In recent years, printed circuit boards are required to exhibit high packaging density of the electronic devices and to be of multi-layered structure, accompanying improvements in performance and downsizing of electronic appliances. A through hole connection method and an inner via hole connection method are known as methods for providing electric interlayer connection between layers of a substrate within which electric connecting among LSIs or other components can be realized at shortest distances.
The through hole connection method, which is employed for general glass reinforced epoxy multi-layered substrates, presents a drawback in that there are restrictions that make it difficult to obtain packaged elements of high density due to the following reasons: electric connection between layers is provided through plating a conductive metal inside through holes so that it is difficult to provide connection only with specific electrodes between layers that are required, and the substrate is generally arranged with a land of electrodes on the uppermost layer of the substrate, a land of electrodes for surface-mounted elements can not be formed on this portion of the land on the side of the substrate.
In order to solve these problems, a method has been proposed in which the number of full through holes can be reduced by making holes extend a half-depth of a laminated substrate. In another method, through holes are blocked on the uppermost layer of a substrate in a process of filling conductive paste with through holes and plating them thereafter to improve the packaging density (e.g. Japanese Patent Publication No. 54-38562; however, their complicated manufacturing processes results in drawbacks in terms of costs and mass production.
In contrast, inner via hole connection is a method in which arbitrary inter-layered electrodes in a multi-layered substrate or printed board are connected at arbitrary positions and is known to provide circuit connection of highest density. The inner via hole connection advantageously makes it possible to connect only specific layers as required, and no through holes may be provided on the uppermost layer of a substrate, and further it is superior in terms of packaging ability (e.g. U.S. Pat. No. 5,481,795, Japanese Patent Application No. 61-191606 (1986)). Applying this method of connection to a resin substrate (for instance, a glass epoxy substrate), a solvent-type silver paste with low viscosity is filled into through holes by printing, which is then dried for curing to continuity. However, the via hole conductors in the obtained substrate present a high specific resistance of approximately 10.sup.-3 .OMEGA..multidot.cm, and it still poses problems in terms of reliability of the conductors or the printed board related to heat impact resistivity such as heat cycle.
In the inner via hole method, via hole conductors are formed by filling conductive paste into via holes through printing, wherein the conductive paste is required to have a low viscosity to enable easy filling. For this purpose, methods have been conventionally taken to decrease the amount of conductive particles in the paste, to employ large conductive particles for decreasing the specific surface area of the particles or to add a solvent or a reactive diluent of low boiling point into the paste (e.g. U.S. Pat. No. 5,326,636).
However, decreasing the amount of conductive filler to be added or employing large particles for obtaining a low viscosity of the paste resulted in lesser contact points of conductive fillers and higher connection resistance of the via holes, and in an environment in which thermal stress such as heat cycle repeatedly generates in the substrate, no reliability of the via hole connection could be secured.
In a method in which a solvent or reactive diluent of low boiling point is added, volatilization of their components during curing with heat press resulted in a large weight loss and drawbacks were presented in that these volatile components caused blisters in the substrate or in that the adhesiveness with connecting copper foils was poor. Further, during many times of printing the paste, only resin components in the paste affixed to printing masks in a film-like manner whereby a composition shift of the paste was caused, and the resulting increase in paste viscosity made continuous printing impossible.